1. Field of the Invention
The present invention relates to a glass antenna installed in or on a window glass of an automobile for receiving a radio frequency signal from a portable radio transmitter for controlling the operation of automotive equipment such as a keyless entry system for automatically unlocking a vehicle door, a similar control system for a luggage compartment lid, a control system for controlling turning on and off of a passenger compartment lamp, etc.
2. Disclosure Information
A keyless entry system for automatically unlocking a vehicle door by receiving a radio frequency signal of a frequency around 40 MHz, 60 MHz, 250 MHz or 300 MHz emitted from a radio transmitter has lately come to attract considerable attention.
In a keyless entry system disclosed in Japanese Provisional Patent Publication No. 62-37475, a pick-up consisting of a loop coil built in a vehicle pillar is used as an antenna. A problem of such a prior art antenna is that it is largely influenced by noise. Another problem is that the antenna is embedded in a vehicle body, so that its tuning cannot be attained with ease.
A glass antenna consisting of conductive strips formed on a lower marginal portion of a side window glass by screen printing has been proposed for use in such a keyless entry system as disclosed in Japanese Provisional Publication No. 63-43403. A problem of this glass antenna is that opening and closing of the window causes variations in its gain, so that during opening of the window the gains in some directions become so small as to cause malfunction of the keyless entry system.
It has further been proposed to install an antenna for a keyless entry system in or on a rear window glass. However, most of the rear window glass is used for installation of the heating element for a defogger, and in may cases an upper marginal part above the heater element or the like part is used for installation of an antenna for receiving TV broadcast waves. For this reason, an antenna for a keyless entry system is subjected to arrangement restrictions and has been incapable of attaining a sufficiently large gain when simply installed in or on a remaining marginal part of the rear window glass.
It has yet further been proposed to install an antenna for receiving FM radio and TV broadcast waves an unoccupied area of a heater element installed in part of a rear window glass as disclosed in Japanese Patent Provisional Publication No. 2-94904 or to connect a feed point of an antenna, which is installed on a marginal part of a rear window glass under a heater element for receiving FM radio and TV broadcast waves, to a bus bar as disclosed in Japanese Patent Provisional Publication No. 1-106502. However, even if such an antenna is used for a keyless entry system, a fall or dip of gain in a particular direction will inevitably be caused, and therefore such an antenna cannot be put into practical use for a keyless entry system.